1. Field of Invention
The present invention relates to a field of micro-electro-mechanical systems (MEMS), in particular to a capacitive acceleration sensor with an “H”-shaped beam and a preparation method thereof.
2. Description of Related Arts
With the progress obtained by the information processing technology and the rapid development of the microprocessor and the computer technology, microprocessors have now been widely used in measurement and control systems. As the capability of these systems strengthens, sensors, as the front end units of information collection systems, play an increasingly important role. Sensors have become crucial parts of automatic systems and robotics. Most broadly, a sensor is a device capable of transforming physical quantity or chemical quantity into an available electrical signal.
An acceleration sensor, just as the name implies, is a sensor element measuring an acceleration value of a moving object, and it is one of the most traditional sensors. According to different detection methods, MEMS acceleration sensors may be divided into capacitive acceleration sensors, piezoresistive acceleration sensors, piezoelectric acceleration sensors, surface acoustic wave acceleration sensors, tunneling acceleration sensors, and the like. A capacitive acceleration sensor comprises a fixed electrode and a movable electrode located on a seismic mass. When the mass block is displaced under the action of an outside acceleration signal, the distance between the movable electrode and the fixed electrode or the area overlapping one another is changed, thereby causing the capacitance value between them to be changed. Via a C/V conversion circuit, it is possible to detect the voltage change which is proportional to the outside acceleration value. The capacitive acceleration sensor has the advantages such as high detection precision, little influence by the temperature changes, and the like.
Capacitive acceleration sensors may be divided into two categories, namely, sandwich capacitive acceleration sensors and comb-finger capacitive acceleration sensors. The sandwich capacitive acceleration sensors may obtain higher detection precision, and the method for manufacturing the sandwich capacitive acceleration sensors is mainly the method of bulk silicon micro-machining. In a sandwich capacitive acceleration sensor, the beam-mass structure whose double sides are entirely symmetrical is the most important sensitive structural part, and its design and manufacturing process is one of the crucial processes of the entire sensor. In the beam-mass structure, if the beam is simply on one surface of the seismic mass and the centroid of the seismic mass and the beam are not on one plane, the transverse acceleration will cause the beam to bend, thereby causing the cross sensitivity of the sensor to rise. Therefore, the manufacturing method of the beam-mass structure whose double sides are entirely symmetrical is very crucial.
In the processing method of the existing sandwich capacitive acceleration sensors, the preparation methods of the sensitive structure of the beam-mass whose double sides are entirely symmetrical include: a concentrated boron-doped self-stop method, a heterogeneous self-stop method and a double layer bonded silicon beam method.
By employing the concentrated boron-doped self-stop method (cf. H Seidel, H Riedel, R Kolbeck, G Mueck, W Kupke, M Koeniger, Capacitive Silicon Accelerometer with Highly Symmetrical Design, Sensors and Actuators A: Physical, Vol. 21, pp. 312-315), when fabricating the entirely symmetrical beam-mass structure, and the process of KOH etching the beam-mass structure as formed finishes, the concentrated boron-doped layer is used as an etching self-stop layer. Thus, the depth of doping decides the thickness of the beam. The disadvantages of such method are that nonuniformity of the doping concentration results in nonuniform thickness of the beam and that the residual stress generated in the boron-doping process will influence the performance of the device, such as sensitivity and linearity, etc.
With respect to fabricating the beam-mass structure whose double sides are parallel and symmetrical, a heterogeneous self-stop method may be used. Take a silicon oxide beam process as an example. The procedure thereof includes manufacturing a pattern of the beam on an oxide layer after oxidizing the silicon wafer, and then releasing the beam-mass structure supported by the silicon oxide beam via silicon etching. As silicon oxide is very crisp and the thickness of silicon oxide obtained by oxidation does not generally exceed 3 μm, the acceleration sensor using a silicon oxide beam has very poor shock resistance.
By employing the double layer bonded silicon beam method, a beam-mass structure whose double sides are parallel and symmetrical is formed (cf. W. S. Henrion, et. al, Sensors structure with L-shaped spring legs, U.S. Pat. No. 5,652,384). The process thereof may employ a method combining KOH etching with Deep Reactive Ion Etching (DRIE). Firstly, KOH is used to etch the silicon wafer from the back to a thickness of the remaining beam, DRIE is then used to release the beam-mass structure from the front, and such beam-mass structure and silicon wafer are further subjected to a bonding process to form a structure whose double sides are entirely symmetrical. Since the entirely symmetrical beam-mass structure can be obtained only by employing a bonding process and a sandwich structure is formed by further performing bonding, the process is very complicated and the cost is comparatively high.